1. Field of the Invention
The present invention relates to an image heat-fixing apparatus installed in an image forming apparatus such as a copying machine, a printer, a facsimile, or a multifunction device, thereof using an electrophotographic system (electrophotographic process technology), and particularly to a safety device in the image heat-fixing apparatus.
2. Related Background Art
A well-known image heat-fixing apparatus in an image forming apparatus includes an electric heating element which is a heat source, a temperature detecting unit for detecting a temperature around a heating temperature, and a control unit for controlling electric power to be supplied from a commercial power source to the electric heating element based on a signal from the temperature detecting unit. The image fixing apparatus is of a thermal type for fixing an image by heating a recording medium, on which an unfixed image is formed and carried, in an imaging section of the image forming apparatus. The foregoing arrangement enables an image fixing temperature to be controlled to a predetermined temperature for image fixing. This type of image heat-fixing apparatus does not function as an image fixing apparatus in case of an abnormal function of at least one of the heating element, the power circuit, the temperature detecting unit, and the control unit. Further, an occurrence of current runaway, if any, may lead to breakdown of the apparatus due to overheating.
Therefore, as disclosed in Japanese Patent Application Laid-Open No. 2008-248813, this type of image fixing apparatus includes a temperature detecting unit such as a thermistor located in the vicinity of the heating element to interrupt electricity supply to the heating element, by means of a current-interrupting unit such as a relay interposed in an energized circuit, in cases where the heating element is abnormally heated. The provision of the safety device prevents overheating, smoking, and fire in case of current runaway. The temperature at which the safety device is activated is set to a level higher than a temperature which is reached during normal operation to prevent the safety device from malfunctioning during normal operation so that the safety device operates only in the case of abnormal overheating.
FIG. 10 shows a block circuit diagram of a conventional safety device. A relay 505 is interposed between a commercial power supply and a heating element to interrupt the electricity supply to the heating element during the period in which the relay 505 is off. An operational amplifier 2009 compares an output level of the thermistor with a predetermined reference level. A reference voltage is generated by dividing a supply voltage Vcc between resistors 2010 and 2011. If the temperature of the thermistor is equal to or less than an abnormal overheating temperature, an output of the operational amplifier 2009 is low. This causes a transistor 2006 to be switched off and further a transistor 2003 to be switched on, by which current flows through a coil in the relay 505 and the relay 505 is switched to a closed position (the state which enables electric power to be supplied to the heating element).
In case of occurrence of current runaway in the above condition, a thermistor detection signal decreases and the output of the operational amplifier 2009 is switched from high to low. This causes the transistor 2006 to be switched on and the transistor 2003 to be switched off, by which the current in the coil in the relay 505 stops and the relay is switched to the open position (the state which disables electric power to be supplied to the heating element). In cases where the relay 505 is switched off, a latch circuit 2013 maintains the off state to prevent the relay from being energized again. If a potential of point G in FIG. 10 becomes low, the latch circuit 2013 fixes the potential of the point G to the low level and thereafter the potential of the point G is maintained at the low level independently of the thermistor condition. A time constant circuit is provided inside the latch circuit 2013, so that the latch operation is activated only after the potential of the point G continues to be low for a predetermined period τ. The reason for the provision of the time constant circuit is to prevent the latch circuit from malfunctioning due to noise in a product which causes the potential of the point G to be momentarily switched to a low level.